In preparing a film-foming polymer for a coating or other kind of curable composition, it is necessary to be able to control the molecular weight of the polymer so that it may be fitted to its particular use or needs. For example, in unperturbed polymerization systems which fundamentally tend to produce high molecular weight polymers, it may be desirable or necessary to limit the molecular weight of the polymers produced, and this must be done in a fairly predictable and controllable fashion. Such molecular weight limitation may be desirable or necessary in the preparation of polymer solutions for use in paints and finishes which require high solids content to assure reduced solvent emission during application and yet which require low viscosity to facilitate ready application.
In free radical polymerizations, there are a number of conventional means of effecting such molecular weight limitation. These, along with notable disadvantages or problems, include:
(1) A high initiator/monomer ratio. However, this may be costly in terms of initiator consumption. Also, high initiator levels may also produce undesirable end groups on the polymers produced.
(2) Polymerization at high temperatures. However, this may lead to undesirable depropagation, thermal initiation, and undesirable secondary reactions.
(3) Adding stoichiometric amounts of thiol chain transfer agents to the polymerizing system. However, the attendant incorporation of sulfur-containing agents into the polymer may render it less durable than is desired. There may also be odor problems associated with the use of sulfur-containing chain transfer agents.
(4) Chain transfer agents employing cobalt (II or III) chelates such as disclosed in U.S. Pat. No. 4,680,352 and U.S. Pat. No. 4,694,054. However, a possible disadvantage of these is that some do not work in water and some are adversely affected or deactivated by low pH. They may cause color problems, especially if interaction with some monomers may require higher levels of the cobalt chelate.
(5) Group transfer agents such as disclosed in U.S. Pat. Nos. 4,417,034 and 4,414,372 or European patent application publication 0 248 596. However, they are specific to methacrylated polymers and relatively expensive. They are also sensitive to certain monomers and impurities which contain active hydrogens, e.g., hydroxyl, carboxyl, carboxylic acid, or amine. Special solvents may be required and blocked hydroxyl and carboxyl monomers may be needed which require a special de-blocking step to activate the protected group.
Terminally or .omega.-ethylenically unsaturated oligomers having chain transfer properties, in some contexts, is also known. There have been a number of studies and articles on such oligomers acting as chain transfer agents.
Such oligomers may be viewed as macromonomers in terms of structure, in that "macromonomers" refer to oligomers which have terminal olefinic moieties. Macromonomer products, however, are usually of a relatively higher molecular weight than such oligomers, although there may be an overlap in terms of molecular weight, particularly where distributions of oligomers and macromonomers are concerned. (A macromonomer may be defined as polymers or copolymers having a terminal olefinic moieties and consisting of about 10 to about 800 monomeric units, with an average molecular weight varying from about 1000 to 50,000, preferably 1,000 to 10,000.)
Terminally olefinic oligomers and macromonomers are known, for example, as disclosed in U.S. Pat. No. 4,547,327; U.S. Pat. No. 4,170,582; U.S. Pat. No. 4,808,656, Japanese patent 3,161,562; Japanese patent 3,161,593. See also, P. Cacioli, et al., J. Makromol. Sci.-Chem., A23 (7), 839-852 (1986) and H. Tanaka, et al., Journal of Polymer Science; Part A; Polymer Chemistry, 27, 1741-1748 (1989).
It is an object of this invention to provide a composition which comprises a functional or crosslinkable film-forming homopolymer or copolymer which has been prepared by employing an .omega.-unsaturated oligomer as a catalytic chain transfer agent.
The present composition can be used in making coatings exhibiting improved properties.
These and other objects will become apparent hereinafter.